1. Field of the Invention
This invention relates to test fixtures for testing printed circuit boards.
2. Prior Art
Test fixtures for printed circuit boards are well known and in extensive use. Typical test fixtures are shown in U.S. Pat. Nos. 4,636,723, 4,746,861, 5,247,249 and 5,430,385.
In such fixtures, a fixed board in a base supports spaced vertical contact probes which are spring biased upwardly and aligned with contacts points on a circuit board which is inserted into the fixture above the test probes. The circuit board extends in a horizontal plane. The printed circuit board is displaced downwardly against the biased probes which are connected to a computer which evaluates the components and connections in the circuit board. A moveable board, which is often referred to as a diaphragm board, since vacuum is used to move such circuit board downwardly, carries the printed circuit board to be tested into contact with the probes.
In one prior art embodiment, the circuit board for test purposes is fixed or clamped to the diaphragm board by a mechanical latch operated manually by the test operator. It is necessary to clamp the circuit board to the diaphragm board, since the total force exerted by the test probes, which are biased upwardly generally by springs, can be substantial, and often in an amount collectively of a force of 600 pounds or more.
In another prior art embodiment, a box forms a vacuum chamber over the printed circuit board. Hold down studs extend downward from the top of the box against the circuit board. Such a box has to be airtight, which prevents access to the components on the board which have to be adjusted, or switches that have to be changed. Additionally, such a construction does not allow for probing with a manual probe.
In summary, in the prior art a printed circuit board is either manually mechanically clamped to a diaphragm board which is then moved by vacuum toward spring biased test probes for contact with the probes and testing of the circuit board, or held by studs within an enclosed vacuum box that prevents easy access to the circuit board.